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Each year on Nov. 11, the U.S. celebrates Veterans Day in honor of those who have fought  and those who have died  for the country. Wreath-laying ceremonies take place at cemeteries across the land, including at Arlington National Cemetery in Virginia. Though the commemoration officially began in Arlington as Armistice Day, with the burial of an anonymous World War I soldier at the Tomb of the Unknowns in 1921, the occasion didn't become a federal holiday in the U.S. until 1938. (In 1954 its name was changed to Veterans Day.) Accounts differ on when the tradition began in Britain and France, but most experts surmise that the first burial of unidentified soldiers at Westminster Abbey in London and at the Arc de Triomphe in Paris took place in 1920, a year before the practice took root in the U.S. (See TIME's pictures "Photographing the Remains of the Fallen.")

The first known ceremony to honor unknown soldiers dates back to the Peloponnesian Wars in ancient Greece, where an empty stretcher was carried in tribute to the dead. Before Armistice Day in 1921, one of the earliest such commemorations in the U.S. was a granite sarcophagus dedicated in 1866 at Arlington in remembrance of the 2,011 unidentified soldiers who died in the U.S. Civil War.

The original unknown soldier buried at Arlington in 1921 was among four who had previously been interred in France. Once the caskets were exhumed, Sergeant Edward F. Younger, a decorated officer, walked around them several times and arbitrarily chose one of the four by placing a handful of white roses upon its top. The coffin lies in a tomb adorned with the phrase, "Here rests in honored glory an American soldier known but to God." In subsequent wars  including World War II, Korea and Vietnam  a solitary unidentified soldier was selected to be honored with an Arlington burial. Other nations have also adopted the ceremony. In Canada, the Tomb of the Unknown Soldier was added to the National War Memorial in Ottawa in 2000, when the casket of a Canadian soldier from World War I was disinterred from a French cemetery and flown across the ocean for burial. Iraq, Australia, Denmark and several countries in South America commemorate their unknown dead in similar ways. (See pictures of the memorial service at Fort Hood.)

The tradition took on great power  in the past, the Pentagon took great pains to ensure the bodies of unknowns remained unidentified, even going so far as to destroy relevant documents about where bodies were discovered and with what, if any, personal effects. But with the advent of DNA testing in the 1980s and '90s, the tradition of burying an unknown soldier has begun to decline. Most soldiers around the world are now required to supply blood samples upon joining the military to ensure their bodies can be identified if they are slain in the line of duty. Although military personnel put their lives at risk for their countries, this requirement, at least, can provide closure to families who might otherwise never be able to lay their loved ones to rest.